Is There a Good Way to Change Many Joined Meshes in Order to Make One Mesh With No Inside?
No garantee it will work but you could try to boolean.
First make sure you don't have holes, select all in Edit mode and Mesh &gt; Clean Up &gt; Fill Holes (set the values in the Operator box):


Then select all your meshes except one and press CtrlF &gt; Intersect (Boolean), and again choose the right settings in the Operator box. If the meshes are closed it should delete all inner vertices: